Cross Polar Discrimination (XPD) of an antenna element or antenna array is a measure of the ability of the array to radiate on its intended polarisation and not into its orthogonal (or cross) polarisation. Radiated power into the orthogonal polarisation (and hence degradation of XPD performance) can be caused by a number of factors including radiation from the corporate feed network, radiation from the element/corporate feed junction/balun network, the fact an X or cross shape is used for the dual polarised element(s) (and hence existence of mutual coupling to a co-incident orthogonal polarised radiating structure within the near field), the chassis, mutual-coupling to other close-by radiating arrays and the fact that wideband operation of an element often implies radiating structure thickness (or departure from pure slant radiating element). Furthermore, antenna arrays which are designed to provide Variable Electrical Tilt (VET) can exhibit varying XPD performance due to varying phase and resulting mutual coupling components between array elements. Similarly, at the mobile terminal, the antenna often needs to have wide or multi-band capability and the existence of proximate physical objects within the near field coupling range of the X-Polar antenna such as a user's hand will also degrade XPD performance.
Therefore, good XPD can prove to be a difficult optimisation exercise when designing antennas for wide bandwidth, multi-band operation and with VET, for base station antennas. Commercial wideband/multi-band VET antennas may achieve <15 dB XPD at boresight that diminishes away from boresight.